Mixin functions (i.e. $bvToast, $bvModal) do not work on Vue 3

Describe the bug

When running with @vue/compat in { MODE: 2 }, it appears that any functions added to Vue instance by a mixin are not available. This includes $bvToast and $bvModal.

Steps to reproduce the bug

1. Run in Vue 3 mode with @vue/compat.
2. Inside of a component, attempt to access this.$bvToast.
3. It does not exist.

Expected behavior

When running with Vue 3, these functions should appear. (It's particularly bad for $bvToast, as there's no other way to dynamically create toasts.)

Versions

Libraries:

• BootstrapVue: 2.23.1
• Bootstrap: 4.6.1
• Vue: 3.2.45
• Vue Compat: 3.2.45

Environment:

• All

Additional context

I'm attempting to upgrade my application to Vue 3 (as an exploratory exercise) and I cannot, for the life of me work out how to use functions added by mixins. Without that, I can't convert the parts of my application that use toasts; they're just broken.